If you shot that image in RAW, you can try to recover some detail using Photoshop, or better yet your camera's native RAW converter.

If the sky is suppoed to be pure blue, you can just do a color replace in photoshop and apply blue, or duplicated the layer, apply blue filter to it, and erase everything else but the sky to show the original image below.

It depends on how badly the sky was overexposed. If it's "blown out" to pure white, the only real option to restore any sort of definition or texture is to bring the image into Photoshop or another editor and create a composite of multiple images - if you have an image with a properly-exposed sky, merge the two images together so that you use the sky from the good image along with the foreground from the image with the blown out sky.

If the sky isn't blown out to pure white but is just overexposed somewhat, you should be able to make some localized exposure adjustments.

When combining with a sky from another photo, make sure that sun is shining from the same direction.
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ImreMay 23 '11 at 20:43

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Keep in mind that the sky is, in general, much brighter than the rest of your scene. An overexposed sky may not fit in the HDR world's definition of "properly exposed" where your histogram is supposed to look like a normal distribution, but a white sky isn't necessarily a bad thing.
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Evan KrallMay 25 '11 at 15:55

What software do you use for post-processing? The highlight recovery tool in Lightroom and Adobe Camera Raw (for Photoshop) does exactly what you're after. (Other software such as Aperture will certainly have a similar feature.) It may be that the sky is so badly overexposed that you simply can't recover any detail, but you'll probably be pleasantly surprised.

Incidentally, this is exactly why you've done the right thing to shoot in RAW: in JPEG mode the camera has already discarded the additional detail in any blown highlights so recovery isn't an option.

If you are using Lightroom, then select the photo, press 'D' to go into Develop Mode. Press 'M' and you will see the cursor over the image changes to a little cross like +.

Move the mouse cursor to the top of the screen, and pressing the left button, draw in from the edge of the photo, down towards the bottom and you will see the graduated filter applied. (It is generally brighter but don't worry we'll fix this). Release the mouse button when the area affected by your filter covers the sky region you wish to fix.

You will notice on the right, you have a series of Mask options. grab the Exposure Slider and slowly move it to the left to apply some negative exposure. You will see the effect in real time on the photo. Stop when you've darkened it enough that you are happy with the effect.

You have applied a graduated ND filter!

Also, you can play with the Recovery slider too, which will help to reduce blown out highlights. These can be made visible by clicking the 'up arrow' to the top RIGHT of the histogram (the up arrow on the top left displays the lost detail in shadows). You might want to play with the recovery slider prior to applying the mask, come to think of it ....

The best way to help with the over exposure is to think about what you are taking a photo of. Is it the sky or your subject? Determine this and also if you are using a DSLR, use a 18% grey card to get exposures. Here is a link that I believe will help you out with your exploration of Photography.
http://en.wikipedia.org/wiki/Gray_card

My preferred way to deal with such a situation is a software emulation of a graduated ND filter. I don't know if all post-processing software has it. It lets you underexspose part of the picture, and you can determine how much of the picture is covered, "rotate" the filter, decide how much you want to underexpose, and give it a tone different from neutral grey (I sometimes use it to give a cyan-ish sky a deeper blue shade). While it doesn't work as well as a real ND filter, it is a good option for when you have to save an already taken picture. Of course, it presumes a largely even horizon - if you have foreground objects protruding into the sky, their protruding parts will be underexposed too, so it doesn't work with all compositions.